1. Technical Field
The present invention relates to methods for repairing nuclear reactor core shrouds. In particular, the method of the present invention is concerned with repair of boiling water reactor shrouds in order to allow safe reactor operation even with extensive cracking along heat affected zones of horizontal welds.
2. Discussion of the Prior Art
Boiling water reactor shrouds are disposed concentrically within the reactor vessel and are commonly formed from multiple arcuate steel plates having a thickness on the order of one and one-half inches. Plates at each horizontal level of the shroud are joined by welds along abutting, relatively short, vertical edges to encircle the reactor at that level. The plates at different adjacent levels are joined along their abutting, relatively long, arcuate edges with horizontal welds. After periods of use, cracking of the shroud tends to occur within heat affected zones of the welds as a result of corrosion, radiation and stress. Cracking of the vertically oriented welds is considered acceptable because these welds are relatively short in length, relative to the overall shroud length, and do not adversely affect the function of the shroud (i.e., support and alignment of the nuclear fuel assemblies, and channeling of reactor coolant flow). Specifically, vertical welds at adjacent levels are offset angularly about the shroud periphery so that cracking of such a weld can extend, at most, only the axial or vertical length of that level. However, if cracking occurs along the longer horizontal or circumferential welds, a crack can extend along the entire circumference or periphery of the shroud, permitting relative lateral movement between the plate levels. Such excessive cracking, therefore, could prevent the core from supporting and aligning the fuel assemblies, can improperly direct or impede coolant flow, and may permit coolant flow leakage.
When excessive horizontal weld cracking occurs, the shroud must either be replaced or repaired. Repair is certainly the preferred alternative in view of the fact that replacement involves significant expense, relatively long shut down time, and the potential for radiation exposure to personnel. To date, however, there has been no acceptable method of shroud repair in situ. Repair techniques used to date typically involve bolting brackets onto vertically adjacent plates across a weld crack. This approach requires plural brackets for each crack, depending upon the length of the crack. Moreover, welds must be separately inspected after repair for additional cracking of the repaired welds as well as for new cracks in other welds.